Characterization of the dissociation behavior of gas-phase protonated and methylated lactones.

The dissociation behavior of gas-phase protonated and methylated four-, five-, six-, and seven-membered ring lactones, some with methyl substituents in various positions, has been characterized by using a quadrupole ion trap mass spectrometer and a triple quadrupole mass spectrometer. The energy dependence of collisionally activated dissociation pathways was determined by energy-resolved mass spectrometry, and the dissociation behavior of the various protonated lactones was compared to that observed for protonated cyclic ketones and ethers of analogous ring size. The protonated cyclic ethers and ketones predominantly dissociated via dehydration, whereas the protonated lactones dissociated via losses of an alkene, ketene, and water. The dissociation behavior of the gas-phase methylated lactones formed from ion/molecule reactions with dimethyl ether ions was compared to the collisionally activated dissociation behavior of isomeric protonated methyl-substituted lactones. The methylation experiments indicated that the gas-phase addition of a methyl group may dramatically alter the favored dissociation pathways when compared to the simple protonated ions.